1. Field of the Invention
The invention relates to a controllable semiconductor component for two current directions having application as control element in electric power supply installations or control circuits, especially as a.c. regulating units.
2. Description of the Prior Art
Such a semiconductor component switching bidirectionally between higher and lower impedance for both current directions with reversible orientation in view of the designations "over" and "under" is known from U.S. Pat. No. 3,275,909. It comprises in a semiconductor wafer a middle zone of a first conductivity type extending through the entire cross-section of the wafer. Adjoining this middle zone above and below are an upper and a lower zone of a second conductivity type opposite to the first. Adjoining a first partial zone on the upper side of the upperzone and a first partial zone on the underside of this lower zone, respectively, are a first and a second main-electrode zone of the first conductivity type. On the upper side of the upper zone of the second conductivity type and separated laterally from the first main-electrode zone is a first control-electrode zone of the second conductivity type. A second control electrode zone of the first conductivity type (called the remote gate) is situated on the upper side of the upper zone at a distance from the first main electrode zone. A first main electrode is in contact with the first main-electrode zone and with a second partial zone of the upper zone. A second main electrode is in contact with the second main electrode zone and with a second partial zone of the lower zone. A control-electrode arrangement for separate contacting of the two control-electrode zones is provided. A case encloses the wafer, where the two control-electrode leads are brought out electrically isolated through the case from their separate control electrodes on the semiconductor chip.
The known semiconductor component thus possesses on one side two control electrodes with ohmic contacts and a main electrode, and on the other side a main electrode. In one design the control-electrode leads are electrically connected to one another, so that a common control-electrode lead is brought out of the case. The known component in this form does not differ externally from a conventional thyristor-triode (Triac) (of e.g. "SCR Manual", 4th edition (1967) General Electric, p. 13; Heumann/Stumps, "Thyristoren" 3rd edition (1947), p. 36) and comprises two thyristors, a normal thyristor for quadrant I of the current-voltage characteristic and an inverted thyristor connected antiparallel to this (quadrant III) in a single semiconductor chip. The inverted thyristor is normally triggered by the "remote gate" somewhat later than the normal thyristor; it is more difficult to fire. In phase-shift control this leads to asymmetry and can thus cause disturbing reactions, e.g. in low-voltage distribution systems.
In another design described in U.S. Pat. No. 3,275,909 the two control-electrode connections and leads are electrically isolated to give a component with four external leads.
The control is already improved with the arrangement, differing from the conventional Triac, of two control electrodes on the semiconductor chip. The first control electrode is contacted on the zone of the second (p-) conductivity type next to the middle zone, in the neighborhood of the first main electrode zone of the first (n-) conductivity type. Laterally, however, it is widely separated from the part of the p-zone which is contacted by the first (there underneath) main electrode, and from the second control electrode (remote gate) which is contacted by a second n-conducting control-electrode zone. In this way the two control electrodes are thoroughly electrically isolated from one another by the long path with high specific resistance in the p-zone, and there also is avoided an electrical short circuit between the control-electrode terminal and the main-electrode terminal. The arrangement of the second control electrode and the second control-electrode zone leads in a known manner to an additional pn-junction for the purpose of indirect-control with negative control current with forward loading of the inverse thyristor or "remote gate thyristor".
The mutual arrangement of two thyristors in a semiconductor chip in the above described form leads, however, to undesirable interaction between the two thyristor systems. When the semiconductor component is rapidly switched between quadrant I and quadrant II minority charge carriers are diffused into the inverse thyristor in the region of the latter's blocking pn-junction even during conduction of the normal thyristor. Upon application of the forward voltage for the inverse transistor, this can lead to turn-on without a corresponding control signal, so that triggering by an undesirably low voltage occurs (DT-AS No. 1,564,420; DT-AS No. 1,931,149; DT-AS No. 2,033,566).
The following measures for solving this problem are known: a) separation of the main-electrode zones (emitter zones) in their projections on a reference plane parallel to the wafer plane (DT-OS No. 1,564,420); b) separation of the emitter zones on the order of at least three diffusion lengths of the minority charge carriers and a larger specific resistance in the region of the semiconductor chip between the two thyristor systems (DT-AS No. 1,931,149); c) binding of the minority charge carriers by recombination centers in the region of the semiconductor chip between the two thyristors systems (DT-AS No. 2,033,566).
In all the above-mentioned known cases Triacs are involved, i.e. bidirectional controllable semiconductor components with three external current leads or external electrode terminals.